1975] 
Tolbert — Avoidance Behavior in Orb Weavers 
33 
A. trifasciata virtually cease construction of barrier webs one to two 
weeks after becoming adults. The omission of barrier webs by ma- 
ture adult females may relate to the increased size and strength of 
the female (size increase is quite rapid at this time), to decreased 
activity of salticids, to the need to maximize food intake before cold 
weather (both sides of the orb are available as catching surfaces 
rather than one side or parts of two sides) , to nerve degeneration or 
some other factor(s). No data are available at this time to determine 
the relative importance of these various factors. 
It is interesting to compare the use of barrier webs by the two 
Argiope species. A. aurantia is the larger of the two and it achieves 
its larger size in a relatively short time. Larger size probably deters 
some salticid and other attacks by invertebrate predators. I did not 
observe such attacks on penultimate and adult A. aurantia while they 
were a frequent occurrence on juvenile A. trifasciata at the same 
time. A. aurantia construct fewer barrier webs and this activity is 
restricted primarily to the mid-instars. A. trifasciata is smaller, 
matures more slowly and constructs barrier webs throughout most 
of its life (Table i). Since the primary barrier excludes some prey 
as well as predators, the inter-relationship between spider size, preda- 
tor avoidance behavior, food intake and web architecture must be 
complex. Factors controlling the building of barrier webs need to 
be determined in order for the niche relations of these two closely 
related, sympatric species to be properly evaluated. 
Stabilimenta 
Stabilimenta, the white zigzag lines of silk which form discrete 
bands in or near the hub of the orb of several spider species, are a 
common feature in Argiope webs. These structures vary considerably 
in form and relative size both between A. aurantia and A. trifasciata 
and within each species. Since I found stabilimenta to possess some 
defensive value to these spiders, their structure and frequency of oc- 
currence are discussed here. 
A. aurantia build extensive disc-shaped stabilimenta as early and 
mid-instars (fig. i). Up to 7 individual bands were often found in 
webs but never more than this and usually less (Table 2). A Cox 
& Stuart test for trend (Conover, 1971) revealed a significant de- 
crease (p<0.05) in the number of bands occurring from mid-instar 
to adult female. This trend is apparent in Table 2. Older spiders 
construct a thinner “patch” of silk at the hub in place of distinct 
stabilimenta bands. Adult females make this patch as well as an 
accompanying linear band below the hub. I also compared adult 
